The determination of the level and temperature of the surface of the burden in a blast furnace (or any other shaft furnace) is well known and the applicants have already advocated the use of equipment for this purpose, this equipment giving very good results, in particularly advantageous conditions from the economic point of view. Such equipment comprises in particular an assembly of deflectors for scanning all or part of the surface to be observed and may be described as follows, with reference to FIG. 1 of the accompanying drawings.